1. Field of the Invention
This invention relates to a radiation image detecting system, and more particularly to a radiation image detecting system which is provided with a two-dimensional image detecting means comprising a plurality of solid radiation detecting elements which are two-dimensionally arranged in an array.
2. Description of the Related Art
There has been known a radiation image information read-out system using radiographic film or a stimulable phosphor sheet in medical radiography.
Recently there has been proposed a radiation image detecting system employing a solid radiation detector including a semiconductor as a major part which detects radiations and converts the intensity of radiations to an electric signal. Though various types of solid radiation detectors have been proposed, the following solid radiation detectors are representative.
That is, a solid radiation detector comprising a two-dimensional image detecting means formed by two-dimensionally forming a plurality of photoelectric conversion elements (each forming a picture element) on an insulating substrate, and a phosphor layer (scintillator) which is formed on the two-dimensional image detecting means and generates visible light bearing thereon image information when exposed to radiations bearing thereon image information. The solid radiation detector of this type will be referred to as xe2x80x9ca photo-conversion type solid radiation detectorxe2x80x9d, hereinbelow.
A solid radiation detector comprising a two-dimensional image detecting means formed by two-dimensionally forming a plurality of charge collection electrodes (each forming a picture element) on an insulating substrate, and a radiation-conductive material layer which is formed on the two-dimensional image detecting means and generates electric charges bearing thereon image information when exposed to radiations bearing thereon image information. The solid radiation detector of this type will be referred to as xe2x80x9ca direct conversion type solid radiation detectorxe2x80x9d, hereinbelow.
The photo-conversion type solid radiation detectors are disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164067, PCT International Publication no. WO92/06501, xe2x80x9cSignal, noise, and read out considerations in the development of amorphous silicon photodiode arrays for radiography and diagnostic x-ray imagingxe2x80x9d, L. E. Antonuk et et. al., University of Michigan, R. A. Street Xerox, PARC, SPIE vol. 1443,xe2x80x9cMedical Imaging Vxe2x80x9d, Image Physics(1991), pp. 108-119, and the like.
In the photo-conversion type solid radiation detector, the photoelectric conversion elements have also function of storing detected electric charges, and the electric charges obtained by photoelectric conversion are stored in the photoelectric conversion elements as latent image charges.
As the direct conversion type solid radiation detector, the following have been proposed.
1) A solid radiation detector which is about ten times as large as normal solid radiation detectors in thickness as measured in the direction in which radiations are transmitted through the solid radiation detector. See xe2x80x9cMATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORSxe2x80x9d, Lawrence Berkeley Laboratory. University of California, Berkeley, Calif. 94720 Xerox Parc. Palo Alto. Calif. 94304.
2) Those comprising a plurality of solid radiation detectors laminated in the direction in which radiations are transmitted with metal plates intervening therebetween. See xe2x80x9cMetal/Amorphous Silicon Multilayer Radiation Detectorsxe2x80x9d, IEE TRANSACTIONS ON NUCLEAR SCIENCE. VOL. 36. NO. 2 APRIL 1989.
3) Solid radiation detectors using CdTe and the like disclosed in Japanese Unexamined Patent Publication No. 1(1989)-216290.
In the direct conversion type solid radiation detector, a capacitor is connected to each of the charge collection electrodes and the electric charges collected by the charge collection electrodes are stored in the capacitors as latent image charges.
This applicant has proposed an improvement on the direct conversion type solid radiation detector as disclosed in Japanese Patent Application 9(1997)-222114 and 10(1998)-215378. The solid radiation detector will be referred to as xe2x80x9can improved direct conversion type solid radiation detectorxe2x80x9d, hereinbelow.
The improved direct conversion type solid radiation detector comprises a first conductive layer which is transparent to recording radiations, a recording photoconductive layer which exhibits photoconductivity upon exposure to the recording radiations passing through the first conductive layer, a charge transfer layer which acts substantially as an insulator to electric charges of the same polarity as that in which the first conductive layer is charged and as a conductor to electric charges reverse to that in which the first conductive layer is charged, a read-out photoconductive layer which exhibits photoconductivity upon exposure to read-out electromagnetic waves, and a second conductive layer which is transparent to the read-out electromagnetic waves. These layers are superposed one on another in this order and latent image charges are collected on the interface between the recording photoconductive layer and the charge transfer layer.
As a system for reading out the latent image charges in the improved direct conversion type solid radiation detector, there may be employed a read-out system where the read-out electrode (the second conductive layer) is like a flat plate and the latent image charges are read out by scanning the read-out electrode with a read-out light spot such as a laser beam, or a read-out system where the read-out electrode is a stripe electrode made up of a plurality line electrodes which extend in one direction and are arranged in another direction and the latent image charges are read out by scanning the stripe electrode with a line read-out beam, extending in a direction perpendicular to the longitudinal direction of the line electrodes, in the longitudinal direction of the line electrodes. Further there has been known a read-out system where a recording stripe electrode made up of a plurality line electrodes is provided so that the line electrodes extend substantially in a direction perpendicular to the longitudinal direction of the line electrodes of the read-out electrode.
In any one of the solid radiation detectors described above, a detecting circuit which detects the latent image charges stored by the solid radiation detecting elements is connected to the solid radiation detector. The detecting circuit converts the latent image charges stored by the solid radiation detecting elements to an image signal, and the image signal is output after subjected to a predetermined image processing, and is reproduced as a visible image by a reproducing system such as a CRT.
The xe2x80x9csolid radiation detecting elementxe2x80x9d is a general term for elements respectively formed by the photoelectric conversion element of the photo-conversion type solid radiation detector and a switching element; by the charge collection electrode of the direct conversion type solid radiation detector, a capacitor and a switching element; and by the charge transfer layer, the read-out photoconductive layer and the second conductive layer of the improved direct conversion type solid radiation detector. In the case of the improved direct conversion type solid radiation detector, the charge transfer layer, the read-out photoconductive layer and the second conductive layer are solid and do not form a plurality of discrete detecting elements. However the parts exposed to the read-out light spot, the parts corresponding to intersections of the line read-out beam and the read-out electrode, that is the line electrodes of the stripe electrode, or the parts corresponding to the intersections of the line electrodes of the recording electrode and the read-out electrode behave as if they were discrete detecting elements. Accordingly, in this specification, it should be interpreted that the parts listed above are also the solid radiation detecting elements and the improved direct conversion type solid radiation detector is also a two-dimensional image detecting means in which a plurality of solid radiation detecting elements are two-dimensionally arranged in a two-dimensional array.
In the photo-conversion type solid radiation detector, radiations impinging upon the phosphor layer is once converted to visible light and charges obtained by detecting the visible light by, for instance, photodiodes are stored as latent image charges. Accordingly, in the solid radiation detector of this type, high frequency components (high sharpness components) of the image information naturally attenuates due to dispersion of light in the course of converting the radiations to visible light, which results in a blurred image.
In the direct conversion type or improved direct conversion type solid radiation detector, when light impinges upon the radiation-conductive material layer or the recording photoconductive layer, electrons (negative charges) and holes (positive charges) are generated. When recording radiation image information on the solid radiation detectors of these types, radiations bearing thereon image information are projected onto the radiation-conductive layer or the recording photoconductive layer while imparting a predetermined direct current field, and charges thereby generated in the radiation-conductive layer or the recording photoconductive layer are collected to the charge collection electrodes or the interface between the recording photoconductive layer and the charge transfer layer by the direct current field, thereby forming latent image charges. Accordingly, in the solid radiation detectors of the direct conversion type or the improved direct conversion type, dispersion of charges cannot occur in the course of generation of charges or collection of the charges and image information can be faithfully reproduced up to a Nyquist frequency, which is determined by the pitches of the array of the solid radiation detecting elements, by separately detecting latent image charges by the respective solid radiation detecting elements, whereby an image can be reproduced with very small blur and with sharpness components reproduced faithfully.
Generally radiation image information includes high frequency components higher than the Nyquist frequency. When radiations bearing thereon radiation image information including high frequency components higher than the Nyquist frequency are projected onto the direct conversion type or the improved direct conversion type solid radiation detector, image signal components bearing thereon the high frequency components higher than the Nyquist frequency turn back toward the low frequency side and are detected as so-called aliasing noise, which increases noise in the image signal.
Since the aliasing noise appears on the low frequency side lower than the Nyquist frequency, it is very difficult to distinguish the aliasing noise from the regular image signal components and to remove the same from the image signal. When an image is reproduced on the basis of an image signal including aliasing noise, an image very bad in quality is output.
To the contrast, in the case of the photo-conversion type solid radiation detector, high frequency components attenuate due to dispersion of light in the course of converting the radiation bearing thereon image information to visible light as described above, and the image signal components bearing thereon the high frequency components higher than the Nyquist frequency also attenuate. Accordingly, in the case of the photo-conversion type solid radiation detector, the problem of deterioration of the image quality due to aliasing noise is less as compared with the direct conversion type or improved direct conversion type solid radiation detector.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image detecting system which can output a high quality image less in aliasing noise even if a direct conversion type or improved direct conversion type solid radiation detector is employed.
In accordance with the present invention, there is provided a radiation image detecting system comprising a solid radiation detector comprising a conversion means which converts radiations bearing thereon image information to electric charges, and a two-dimensional image detecting means which detects the electric charges obtained by the conversion and is formed by a two-dimensional array of a plurality of solid radiation detecting elements, each corresponding to a picture element,
wherein the improvement comprises a high frequency component attenuation means which attenuates high frequency components of the electric charges bearing thereon high frequency components of the image information not lower than a Nyquist frequency, which is defined by the pitches of the array of the solid radiation detecting elements, so that aliasing noise due to the high frequency components of the image information not lower than the Nyquist frequency becomes not stronger than 30% of intrinsic noise power at a frequency equal to a half of the Nyquist frequency.
The xe2x80x9cconversion means which converts radiations bearing thereon image information to electric chargesxe2x80x9d is the recording photoconductive layer when an improved direct conversion type solid radiation detector is used, and is the radiation-conductive layer when a direct conversion type solid radiation detector is used.
The xe2x80x9cpitches of the array of the solid radiation detecting elementsxe2x80x9d are pitches at which the xe2x80x9csolid radiation detecting elementsxe2x80x9d as defined above are arranged in an array.
The xe2x80x9cintrinsic noise powerxe2x80x9d means the power which the aliasing noise would have but for the high frequency component attenuation means.
The high frequency component attenuation means is a means for attenuating the high frequency components of the electric charges bearing thereon high frequency components of the image information not lower than a Nyquist frequency and may be of any form so long as it can attenuate the high frequency components so that the aliasing noise due to the high frequency components of the image information not lower than the Nyquist frequency becomes not stronger than 30% of intrinsic noise power at a frequency equal to a half of the Nyquist frequency. For example, the high frequency component attenuation means may attenuate the high frequency components physically, chemically, optically, electrically or magnetically. In the case where the high frequency component attenuation means attenuates the high frequency components physically or chemically, the attenuation may be realized by adjusting the material or the thickness of the recording photoconductive layer and/or the radiation-conductive layer. In the case where the high frequency component attenuation means attenuates the high frequency components optically, the attenuation may be realized by once converting the radiations to visible light. In the case where the high frequency component attenuation means attenuates the high frequency components electrically, the attenuation may be realized by controlling the electric field applied to the recording photoconductive layer and the radiation-conductive layer, and in the case where the high frequency component attenuation means attenuates the high frequency components magnetically, the attenuation may be realized by applying a magnetic field to the recording photoconductive layer and the radiation-conductive layer.
However it is preferred that the high frequency component attenuation means comprises a phosphor layer which generates visible light upon exposure to radiations bearing thereon the image information and is provided on the conversion means side of the solid radiation detector and the conversion means generates electric charges upon exposure to the visible light generated by the phosphor layer. The degree of the attenuation may be controlled by controlling the material and/or the thickness of the phosphor layer taking into account the pitches of the array of the solid radiation detecting elements, and the thickness and/or the material of the conversion means.
It is further preferred that the phosphor layer be removable, or the thickness and/or the material of the phosphor layer be variable.
In the radiation image detecting system of the present invention, since the high frequecy component attenuation means attenuates the high frequency components of the electric charges bearing thereon high frequency components of the image information not lower than the Nyquist frequency so that aliasing noise due to the high frequency components of the image information not lower than the Nyquist frequency becomes not stronger than 30% of intrinsic noise power at a frequency equal to a half of the Nyquist frequency, the aliasing noise included in the image signal is reduced and a high quality image less in the aliasing noise can be obtained.
When the high frequecy component attenuation means comprises a phosphor layer which generates visible light upon exposure to radiations bearing thereon the image information and is provided on the conversion means side of the solid radiation detector and the conversion means generates electric charges upon exposure to the visible light generated by the phosphor layer, the high frequency components in the image information can be attenuated due to dispersion of light in the course of converting the radiations to visible light and the present invention can be realized with a simple structure.
Further when the phosphor layer is removable, the phosphor layer can be removed, for instance, when it is desired that the high frequency components be faithfully reproduced.
Further when the thickness and/or the material of the phosphor layer is variable, requirement on the quality of the output image can be satisfied while keeping a balance of aliasing noise and reproducibility of the high frequency components.